1. Field of the Invention
The present invention relates to a multicolor image forming apparatus. More particularly, the present invention relates to a multicolor image forming apparatus suitable for the prevention of streak-like image defect caused by vibration accompanying the operation of a development unit.
2. Description of the Related Art
In a multicolor image forming apparatus, Y (yellow), M (magenta), C (cyan) and K (black) toner images are formed on a first image carrier. These color toner images are primarily transferred onto a second image carrier so that they are superimposed on each other to form a multicolor toner image. The multicolor toner image thus formed is then secondarily transferred onto a printing medium such as paper and plastic film sheet to obtain a desired multicolor image. The foregoing monochromatic toner images are each obtained by irradiating the first image carrier with laser beam modified according to image data to form an electrostatic latent image, which is then developed by a development unit for each color. The development unit is provided for each color. These development units are sequentially arranged opposed to the first image carrier to effect development. As an example of a development device, there is known a rotary development device.
In a rotary development device, respective color development units are equally disposed on the circumference of the rotary device. These color development units are rotated by an angle of 90 degrees and positioned corresponding to the development timing of the respective colors to effect development. During development, a development roll to which a toner has been attached is arranged opposed to the first image carrier at a predetermined gap. An apparatus is known which is arranged to allow a tracking roll as a gap holding means to come in contact with the surface of the first image carrier to keep the gap between the development roll and the first image carrier constant.
An apparatus can be proposed which is arranged to keep the gap between the development roll and the first image carrier at a proper value by allowing the tracking roll to come in contact with a separate member properly positioned relative to the first image carrier rather than allowing the tracking roll to come in contact with the first image carrier. As an example of an image forming apparatus there is disclosed one including a rotary development device in Japanese Patent Unexamined Publication No. Hei. 5-188829.
The foregoing conventional image forming apparatus has the following problems. Firstly, the image forming apparatus which allows the tracking roll to come in direct contact with the first image carrier is disadvantageous in that when the development roll is rotated during positioning, an impact vibration is transmitted to the first image carrier. If the first image carrier is irradiated with laser beam for the subsequent image formation while the development roll is being positioned, the foregoing impact causes electrostatic latent images to be shifted, generating band-like streaks (banding) or other image defects.
The image forming apparatus which allows the tracking roll to come in contact with the member other than the first image carrier is not liable to the transmission of impact to the first image carrier. However, since this mechanism involves indirect positioning, it is not necessarily easy to keep the gap between the development roll and the first image carrier accurate. In other words, this mechanism is disadvantageous in that the provision of the member other than the first image carrier adds to the number of parts required. This mechanism is also disadvantageous in that the accurate adjustment of the position of the member other than the first image carrier and the image carrier relative to each other requires an increased number of assembling steps.